The present invention relates generally to plastic netting for wrapping articles, and more specifically to a particular form of plastic netting for use in wrapping palletized loads and other multiple package systems, with improved results.
The wrapping of palletized loads in order to unitize the load is well known. It is taught, for example, in U.S. Pat. Nos. 3,495,375; 3,867,806 and 4,067,174, to utilize plastic films to wrap such loads.
The use of plastic netting for wrapping palletized loads also is known in the art. The use of such netting is disclosed, for example, in U.S. Pat. Nos. 3,945,493 and 4,136,501 (Re. 31,191). The former patent teaches a process in which the net is heat shrunk about the load, while the latter patent teaches a process in which the net is stretched as it is being applied under tension. U.S. Pat. No. 4,206,846 relates generally to similar subject matter for netting used in a pallet wrapping operation.
U.S. Pat. Nos. 4,208,457 and 4,332,326, disclose the use of netting in which the strands are molecularly oriented before the net is applied to the load.
U.S. Pat. No. 4,402,409 also is directed to improved netting for use in a pallet wrapping operation, in which the netting has a relatively low degree of permanent stretch and a relatively high degree of elastic stretch.
All of the plastic netting used to heretofore and referred to above in pallet wrapping operations has been of the "square mesh" configuration or "bias configuration". Although U.S. Pat. Nos. 4,332,326 and 4,402,409 make reference to "diamond mesh" configuration netting, they specifically teach cutting the formed, tubular netting in a manner to convert the netting into a "square mesh" net, before the net is used in a pallet wrapping operation.
The "square mesh" and the "bias configuration" nets of the prior art consist of longitudinal and transverse strands arranged at substantially right angles to each other. When these nets are used in pallet wrapping operations, the longitudinal strands, i.e., the strands extending in the direction of wrap (typically the machine direction of web formation) are required to sustain most of the tension, due to the fact that the tension forces are imposed along the longitudinal direction in which the machine direction strands extend. The transverse strands actually provide little or no support to the load. In fact, these transverse strands tend to buckle and become limp when the longitudinal strands are tensioned during a pallet wrapping operation. Thus, despite the fact that a load might be tightly wrapped by means of the tensioning of the longitudinal strands, the relatively loose transverse strands tend to permit the load to shift to a degree which can be unacceptable. This is particularly true when the transverse strands bridge a pair of articles stacked upon each other on a pallet or other support structure.
U.S. Patent No. 4,741,442, covers an earlier invention by me, which, at the time of filing, I believed to overcome the above-discussed deficiencies of the "square mesh" and "bias configuration" netting. Specifically, in the '442 patent I described and claimed a plastic netting for palletized loads, wherein diamond-shaped apertures were formed by strands of the netting located at an acute angle to the longitudinal direction in which the web was to be tensioned for wrapping a palletized load. It was believed that the strands defining the diamond-shaped apertures, by virtue of the fact that they were all disposed at an acute angle to the tensioning direction, would become equally tensioned, along the entire extent of the net.
During subsequent experimentation I discovered that the netting described and claimed in the '442 patent actually had greater deficiencies than the prior art "square mesh" and "bias configuration" products. Specifically, the "diamond-shaped" netting forming the subject matter of the '442 patent had two significant problems, when used under tension as a wrap for a palletized load.
First, the netting necked-down excessively, i.e., to approximately 20%-25% of its initial transverse dimension. Specifically, a netting having an unstretched, transverse dimension of approximately 20 inches was observed to neck-down to approximately 4 or 5 inches, under the tension forces normally applied to the netting during a pallet-wrapping operation; e.g., 1-5 pounds force per inch of width.
Second, the sections of the netting adjacent the longitudinal edges were in a limp, floppy, untensioned condition under machine direction loading of the type encountered in a pallet-wrapping operation. Although the longitudinal center region of the netting did become taut, due predominately to a shifting or change in alignment of the strands, as opposed to substantial stretching of the strands in that region, the strands in the floppy end regions did not realign, and therefore were incapable of contributing to any significant degree to preventing a shifting of a palletized load. Thus, contrary to the statements in my prior '442 patent, all strands of the diamond shaped netting were not subjected to approximately equal tension when wrapped about a pallet load.
In order to compensate for the inadequacy of the floppy ends to constrain a palletized load, when employing the netting disclosed in the '442 patent, it would be necessary to provide a substantial degree of overlap between adjacent wraps of the netting, whereby floppy edges of each wrap would be covered by taut central sections of the adjacent wraps. Unfortunately, this results in the use of an excessive quantity of material, thereby resulting in undesired costs.
All of the patents referred to herein are incorporated by reference. Reference to these patents will disclose the wrapping methods used in pallet wrapping operations.
The U.S. Pat. No. 4,594,280 to Coyon et al., discloses an expanded perforated sheet having unexpanded sides 4. The structure is disclosed as being usable as an electrical resistance element, not as a flexible web of wrapping material.
The U.S. Pat. Nos. 3,040,968 to Long et al., 3,550,842 and 3,603,369, Scholz, and Bruno, 3,762,629, disclose wrappers provided with apertured central regions surrounded by unapertured margins. There is no disclosure of providing the edge regions with a different degree of elasticity than the strand regions defining the apertures, for the purpose of transmitting tension forces to the strands defining the apertures.
The U.S. Pat. No. 4,303,717, to Mercer, discloses a plastic netting having apertures 7 and integrally formed selvedge 17. Mercer states that the stretch in the edge zone 17 is not substantially greater than that of the zones between the immediately adjacent holes or depressions 7. There is no disclosure of forming the selvedge or edge zones 17 to be more elastic than the strands defining the depressions 7, for transmitting tension forces to said strands.